Method for producing layered structures on a substrate, substrate and semiconductor components produced according to said method

ABSTRACT

The invention relates to a method of manufacturing layer-like structures in which a material layer having hollow cavities, preferably a porous material layer, is produced on or out of a substrate consisting, for example, of monocrystalline p-type or n-type Si and in which the layer-like structure, or a part of it, is subsequently provided on the cavity exhibiting or porous material layer. The layer-like structure, or a part of it, is subsequently separated from the substrate using the layer having the hollow cavities, or porous layer, as a point of desired separation, for example through the production of a mechanical strain within or at a boundary surface of the cavity exhibiting or porous layer. The method is characterised in that the surface of the substrate is structured prior to the production of the porous layer, or in that the surface of the porous layer is structured.

[0001] The present invention relates to a method of manufacturinglayer-like structures in which a material layer having hollow cavities,preferably a porous material layer, is produced on a substrateconsisting, for example, of monocrystalline p-type or n-type Si and inwhich the layer-like structure, or a part of it, is subsequentlyprovided on the cavity exhibiting or porous material layer, and issubsequently separated from the substrate using the cavity exhibiting orporous layer as a position of intended separation, for example throughthe generation of a mechanical strain within or at a boundary surface ofthe cavity exhibiting or porous layer. Furthermore, the inventionrelates to different substrates which can be produced by this method andto novel semiconductor components which can be manufactured using thesubstrates of the invention.

[0002] A method of the initially named kind is known from severaldocuments.

[0003] By way of example, a method of manufacturing a semiconductor bodyis described in the European patent application with the publication no.0 528 229 A1 in which a silicon substrate is made porous, a non-porous,mono-crystalline silicon layer is formed on the porous silicon substrateat a first temperature and in which a surface of the non-porous,monocrystalline silicon layer is bonded to a second substrate having aninsulating material at its surface. Thereafter the porous silicon layeris removed by a chemical etching process and a further monocrystallinesilicon layer is grown onto the first named non-porous monocrystallinesilicon layer by an epitaxial process at a second temperature.

[0004] The sense of this method is to be able to grow monocrystallinesilicon on any desired substrate. The method is, however, relativelycomplicated because the porous silicon layer must be etched away.Similar methods are also apparent from the European patentspecifications with the publication nos. 0 536 788 A1 and EP 0 449 589A1.

[0005] In EP-A-0 767 486 a method of the initially named kind isdescribed in which the porous layer has a region of increased porosityand the separation of the layer-like structure from the substrate takesplace by mechanical separation in the region of increased porosity. Theregion of increased porosity is produced either by ion implantation orby a changed current density during the manufacture of the porous layer.Even if the method step of the separation can be improved hereby, themethod is more complicated and an increased danger exists of undesiredseparation prior to or during the production of the layer-likestructure. A multiple use of the starting substrate is admittedlyachieved here, nevertheless one would, in many potential uses, use theexpensive monocrystalline substrate in a relatively wasteful manner.

[0006] A similar proposal can be found from the non-prior publishedEP-A-0 797 258.

[0007] The manufacture of silicon solar cells at favourable costrequires high quality silicon, as far as possible single crystal siliconfor high photo voltages, thin Si-layers for material saving, butnevertheless adequate absorption, low manufacturing temperatures forenergy saving and favourably priced foreign substrates, for exampleglass for the mechanical stability.

[0008] So far as is known, there are no methods which satisfy all thesecriteria. For example, work is described in some of the above namedEuropean patent applications in which one carries out CVD epitaxy attemperatures above 800° C. on porous silicon and transfers the so-formedepitaxial layers to a glass substrate. The silicon layers are notstructured. For the separation, wet chemical processes or processeswhich destroy the substrate wafer are used. Applications in thephotovoltaic field are not discussed.

[0009] The paper “Ultrathin crystalline silicon solar cells on glasssubstrates” by Rolf Urendel, Ralf B. Bergmann, Peter Lötgen, MichaelWolf and Jürgen H. Werner, which appeared in Appl. Phys. Lett. 70(3),Jan. 20, 1997, describes a possibility of manufacturing structuredpolycrystalline silicon layers which are suitable for use as photocells.The paper is, however, not concerned with single crystal material andrequires a complex structuring of the glass substrate and also a complexcontacting of the p- and n-layers in order to realise the photocell.

[0010] Further documents which are concerned with porous silicon fordifferent purposes include publications from the Research Centre Jülich,which deal with the manufacture of lateral diffraction gratings on thebasis of porous silicon and interference filters of porous silicon. Thepaper “Optical sensors based on porous silicon multi-layers: Aprototype” by W. Theiβ, R. Arens-Fischer, S. Hilbrich, D. Scheyen, M. G.Berger, M. Krüger, M.

[0011] Thönissen, gives further information concerning the manufactureof porous silicon structures and possible applications of theso-produced structures. Thin layer silicon solar cells are moreoverdescribed in the publication “Crystalline thin film silicon solar cellsby ion-assisted deposition” by S. Oelting, Dr. Martini and D. Bonnet.This publication appeared on the occasion of the “Twelfth EuropeanPhotovoltaic Solar Energy Conference” from Apr. 11-15, 1994.

[0012] The object of the present invention is to propose a method of theinitially named kind which overcomes the above named problems andenables the manufacture of components, in particular, but not only,silicon solar cells at favourable cost, high quality silicon, so far aspossible single crystal silicon, for high photovoltages and thin siliconlayers for material saving, but simultaneously achieving enhanced lightabsorption while using low manufacturing temperatures and costfavourable foreign substrates. In particular a method is aimed at inwhich the substrate used can be reused, or in which a plurality of likestructures can be produced at favourable cost.

[0013] It is also an object of the present invention to propose methodsfor the production of different novel substrates which form the startingpoint for the production of further structures by means of epitaxialmethods. Moreover, it is an object of the present invention to provide aphotocell and other semiconductor components using the method of theinvention which can be manufactured at favourable cost and which haveexcellent technical characteristics.

[0014] For the solution of this object methodwise, provision is made inaccordance with the invention that the surface of the substrate isstructured prior to the production of the porous layer or that thesurface of the porous layer is structured.

[0015] Since the surface of the porous layer is structured, themechanical separation at the boundary surface to the layer-likestructure can apparently be carried out better, without need to produceporous layers with two different porosities. However, not only amechanical separation comes into question, but rather also other methodswhich will be explained in more detail later.

[0016] Particularly important are the savings of time, effort andmaterial which can be achieved by the use of structured layers, andindeed in particular when the structuring is exploited in the finalproduct. Since the porous layer has a corresponding surface structuring,the layer-like structure can be provided with the same structuring.

[0017] In the manufacture of thin components with structured surfaces,only thin layer-like structures need then be produced. If one, however,operates in accordance with the prior art, which aims at planarsurfaces, it is first necessary to produce thicker layers which mustthen be structured in a complicated manner by the removal of material.

[0018] That is to say, using the method of the invention, the porouslayer can be made relatively thin, preferably in the range of about 100nm to 10 μ, so that relatively little material is lost and the workingspeed is improved, particularly since the once produced surfacestructuring of the substrate can be exploited for the manufacture of aplurality of identically structured layer-like structures.

[0019] When the separation of the layer-like structure from thesubstrate is carried out using mechanical stresses, then this separationtakes place methodwise by means of the invention with structuredsurfaces in such a way that only the porous layer is damaged, but notthe substrate or the layer-like structure. In many cases it is possibleto carry out the separation at the upper boundary surface of the porouslayer remote from the substrate so that the porous layer remainspreserved. Accordingly, it is easy to reuse the substrate. For this, theporous layer is first removed, since it is as a rule damaged. A newporous layer is then produced on the substrate, after freeing it fromthe remainder of the porous layer, whereby the substrate can be reused.This is not possible in the prior art, when the porous layer is removedby etching or by mechanical removal from the layer-like structure.

[0020] It should be stated at this point that it would be possible toachieve this position of intended fracture or surface of intendedfracture by a layer having hollow cavities instead by a porous layer,with it, for example, being possible to produce the hollow cavities byphotolithography and for the hollow cavities to be open towards the freesurface of the substrate. In this application only porous layers willnow be discussed for the sake of simplicity. It should, however, beunderstood that these also include layers having hollow cavities andforming positions of intended fracture.

[0021] This type of separation of the layer-like structure from thesubstrate also succeeds when the surface of the porous layer is madeflat. It is particularly favourable, in particular for the manufactureof the photocells or various other components, when the surface of theporous layer remote from the substrate is structured, since, on growingthe layer-like structure on the porous layer, the layer-like structurereflects the structuring of the porous layer, so that, for example witha solar cell, the trapping of light takes place with substantiallyhigher efficiency.

[0022] Since the structured surface of the substrate is preserved andcan be reused, optionally after a cleaning step or after freshening upthe structuring, a plurality of identical, layer-like structures can bemanufactured from one substrate, which substantially increases theeconomy of the method, particularly since it is not necessary to newlystructure the substrate every time.

[0023] The production of the structured surface of the porous layer can,in principle, take place in two ways. On the one hand, one can structurethe surface of the single crystal substrate and then make it porous inmanner known per se. The manufacturing process for the porous layer thenautomatically leads, with thin layers, to a porous layer having the samestructuring as the structured substrate itself at its upper boundarysurface remote from the substrate and at its lower (complementary)boundary surface facing the substrate. As an alternative to this, theplanar surface of the single crystalline semiconductor substrate can bemade porous, and the surface of the porous layer can be subsequentlystructured. Various possibilities for carrying out this structuring areset forth in the claims 2 and 3.

[0024] The substrate need not necessarily be monocrystalline, but canalso be polycrystalline—assuming that the grain sizes of thepolycrystalline material are larger than the width and thicknessdimensions of the structuring, and the thickness of the porous layer,for example grain sizes of 100 μm to centimetre size.

[0025] The typical structurings which come into consideration for solarcells have thickness and width differences which each lie in the rangefrom 0.5μ to 100 μ. Through the use of thin porous layers in the rangefrom approximately 100 nm to 10μ, the shape of the porous surface of theporous layer remains true to the structured shape of the substrate, evenwith multiple use of the same, i.e. also with multiple production of theporous layers on one and the same substrate.

[0026] The layer-like structure is at least partly applied by anepitaxial method to the porous surface. The porous layer namely has thesame crystalline structure as the original substrate and is well suitedfor the growth of layer-like structures by means of epitaxial methods,with the so grown layer-like structure then having the same crystallinestructure, i.e. they are also monocrystalline.

[0027] The epitaxial method can be carried out as a homo-epitaxialmethod or as a hetero-epitaxial method. With hetero-epitaxy it isfavourable that the porous layer can yield somewhat, so that apronounced strain in the boundary surface region need not be feared.

[0028] Through the epitaxial method at least one semiconductor layerbelonging to the layer-like structure is applied onto the surface of theporous layer.

[0029] Depending on the purpose for which the layer-like structure isintended, other layers can then be applied onto the so producedsemiconductor layer, with it not being compulsory for these furtherlayers to likewise have a monocrystalline structure. However, there aremany structures in which the layer-like structure will consist of aplurality of monocrystalline semiconductor layers, for example twolayers which form a p-n-junction.

[0030] It is, however, also possible, in accordance with the invention,in accordance with claims 5 and 6, to deposit a metal layer onto thelayer-like structure and/or to apply a dielectric, for example in theform of a transparent or light-permeable window layer, for examplethrough the Sol-Gel process or by means of an adhesive.

[0031] It is particularly favourable if a carrier layer is providedwhich is either brought into contact with the layer-like structure, forexample by adhesive bonding, by wafer bonding or by a diffusionsoldering process, or is formed as part of the layer-like structure, forexample through a continuation of the epitaxial process. If the carrierlayer is applied onto the surface of the layer-like structure byadhesive bonding, by wafer bonding or by a diffusion process, then itcan, for example, consist of glass or aluminium. This carrier layer ofthe carrier will normally consist of a favourably priced and stablematerial, for example of glass. The mechanical separation of thelayer-like structure from the substrate can then take place in that one,for example, pulls on the carrier layer or on the carrier, so that thecarrier layer or the carrier with the layer-like structure separatesfrom the substrate. The carrier layer or the carrier then forms afurther substrate, on which the layer-like structure is provided. Onecan now carry out further method steps on the free surface of thelayer-like structure. For example, if the layer-like structurerepresents a finished semiconductor element, this can simply be coveredover or provided with a passivation or with surface contacts. This is ofexceptional importance, because one can, by means of the invention,produce contacts, gates or electrodes on both sides of the layer-likestructure, which brings many advantages both from a technicalmanufacturing point of view as well as with regard to the physicalcharacteristics of the so produced semiconductor components.

[0032] In the event that the layer-like structure is not yet finished,one can produce further semiconductor layers by epitaxial methods on thefree surface of the layer-like structure and can optionally also effectfurther structuring by photolithographic methods or other methods, in sofar as this is necessary. The crystalline structure of the layer-likestructure is then retained during the further course of the epitaxialmethod.

[0033] As initially mentioned, the substrate with the remainder of theporous layer can then be used anew, after the separation of thelayer-like structure from the substrate at the point of intendedfracture that is provided, as a substrate for the application of afurther layer-like structure.

[0034] The method can be particularly favourably further developed inaccordance with claim 12 in that a further porous layer is produced onthe surface of the layer-like structure remote from the substrate priorto or after the separation of the layer-like structure from thesubstrate, and a further layer-like structure can be provided on thefurther porous layer, with the method optionally being repeatableseveral times, whereby a plurality of layer-like structures, inparticular structured layer-like structures, arise above one another,which are respectively separated from the adjacent layer-like structureby a porous layer forming a position of intended fracture, wherein,after the production of such a multiple structure, the individuallayer-like structures can be separated from one another by theproduction of a mechanical stress within or at a boundary surface of therespective porous layer.

[0035] Through the production of the so described multiple structure, avery rational manufacture of individual layer-like structures isachieved, which can then be separated one after the other from themultiple structure, which takes place best of all in accordance withclaim 14. That is to say, prior to the separation of the individuallayer-like structures from the multiple structure, they are eachprovided with a carrier layer or are secured to a carrier, precisely asis the case when a single layer-like structure is formed on thesubstrate, as described in more detail above.

[0036] In this variant of the method, further structures can also beoptionally grown by epitaxial methods on the free surfaces of the soformed layer-like structures.

[0037] An alternative variant of the method of the invention ischaracterised in that one generates or applies a porous material layerout of or onto a first substrate, the layer optionally having astructured free surface, for example having grooves arranged parallel toone another, in that one applies a second substrate onto the freeoptionally structured surface of the porous material layer andsubsequently separates the second substrate from the first substrateusing the porous layer as a position of intended fracture by theproduction of a mechanical strain such that a layer or sections of theporous material layer remains or remain adhered on the second substrate,whereby the second substrate can be used for epitaxial methods.

[0038] It is particularly favourable if, after the separation of thesecond substrate from the first substrate, the residue of the porouslayer is removed from the first substrate, a new porous layer isproduced on the substrate and the above process is repeated, with thisprocess being optionally repeatable a plurality of times in order toproduce a plurality of second substrates starting from a firstsubstrate.

[0039] Since the sections of the porous material layer remain bonded toeach second substrate, any desired layer-like structures can be grownonto these substrates by means of the epitaxial methods. Since thealignment of the crystal structure in each section of the porousmaterial layer is the same, the structures grown on the secondsubstrates by epitaxial methods likewise have a monocrystallinestructure, so that one can, starting from an expensive substrate,produce a plurality of substrates for epitaxial methods in a favourablypriced manner.

[0040] Various possibilities exist for applying the second substrateonto the first substrate. One possibility lies in using anadhesive—another possibility would be to deposit a metal layer onto theporous surface of the first substrate and to then connect this metalliclayer to a carrier material in a different manner. A carrier materialcan also be connected to the porous layer of the first substrate bymeans of a diffusion brazing process. It is only important that afterthe removal of the second substrate, sections of the porous material ofthe first substrate are present in distributed manner on the surface ofthe second substrate.

[0041] A further interesting possibility for manufacturing substratematerial with a porous layer can be found in claim 20.

[0042] Various possibilities exist for the production of mechanicalstress within the porous layer, which leads to the separation of thelayer-like structure or a part of it from the substrate. Thesepossibilities are set forth in claim 22.

[0043] The substrates produced by the method are intermediate products,which are valuable in themselves, and they are set forth more preciselyin the claims 26 to 35.

[0044] Further variants of the invention which are interesting inpractice can be found in the claims 23 and 24.

[0045] The method of the invention is in particular used for themanufacture of high quality solar cells, which are claimed per se inclaim 36. A further possible use is a radiation detector, which isclaimed in claim 39.

[0046] Preferred embodiments of the subject matter of the invention areset forth in the subordinate claims.

[0047] The invention will be explained in more detail in the followingwith reference to embodiments and to the drawings, which show:

[0048]FIGS. 1A to 1F

[0049] a sequence of sketches to illustrate a first variant of themanufacturing method of the invention,

[0050]FIG. 2

[0051] an electron microscope recording of a layer-like structure,corresponding to FIG. 1B without carrier layer,

[0052]FIG. 3

[0053] an electron microscope recording of the top side of the substrateafter the removal of the layer-like structure of FIG. 2, but prior tothe carrying out of the cleaning step,

[0054]FIG. 4

[0055] an electron microscope recording of the layer-like structure ofFIG. 1 from a different viewing angle in order to document the qualityof the lower side of the layer-like structure,

[0056]FIG. 5

[0057] a schematic illustration of a possible multiple structure whichcan be produced by means of the method of the invention,

[0058]FIGS. 6A to 6C

[0059] schematic illustrations of a variant of the method of theinvention,

[0060]FIG. 7

[0061] a further variant of the method of the invention,

[0062]FIG. 8

[0063] a schematic cross-section through a solar cell produced inaccordance with the method of the invention,

[0064]FIG. 9

[0065] a plan view of the structure of FIG. 8 in the plane IX-IX,

[0066]FIG. 10

[0067] a schematic cross-section through a radiation detector,

[0068]FIG. 11

[0069] a representation of the detector of FIG. 10 as seen in the freedirection XI,

[0070]FIGS. 12A to D

[0071] a schematic representation similar to FIG. 6, but withmodifications,

[0072]FIGS. 13A to H

[0073] sketches to illustrate the manufacture of a semiconductor layerwhich is monocrystalline in certain regions and amorphous in others,

[0074]FIG. 14

[0075] X-ray diffraction spectra of the silicon “waffle” of FIG. 2 andof the monocrystalline substrate,

[0076]FIG. 15

[0077] transient microwave reflectivity ΔR of the Wf=5.8 μm thick Si“waffle” of FIG. 2 after optical excitation with a 20 ns laser pulse,

[0078]FIG. 16

[0079] the measured hemispherical reflectivity of the encapsulatedwaffle structure,

[0080]FIG. 17

[0081] the theoretical energy conversion efficiency (solid lines) andideal cell thickness (broken lines) for solar cells with the wafflestructure of FIG. 2,

[0082]FIGS. 18, 19

[0083] a schematic illustration similar to FIGS. 8 and 9, but with amodified embodiment,

[0084]FIG. 20

[0085] the series connection of solar cells in the module,

[0086]FIG. 21

[0087] a schematic representation of the integrated connection throughthe use of a shadow mask, which is moved during the Ψ process into thepositions 1 to 3, with the mask or the sample being shiftedhorizontally,

[0088]FIGS. 22A to 22E

[0089] sketches to explain a method in accordance with the invention forthe separation of the layer-like structure from the substrate in theregion of the boundary surface to the porous layer.

[0090]FIG. 1A shows a silicon substrate 10, for example of p-Si, with ann-Si substrate likewise entering into question. The one surface of theSi substrate 10 has a structuring 12 which one can consider as a matrixof pyramidal recesses 14, the base surfaces of which are placed directlyalongside one another, so that the upper boundary of the surface is verysimilar to a square grid.

[0091] The substrate 10 is subsequently treated in manner known per sein order to produce a porous silicon layer 18 (FIG. 1B). The upper sideof the porous silicon layer 18 has the same shape as the structuredsurface of the Si substrate 10. The boundary surface between the poroussilicon layer 18 and the substrate has the same shape.

[0092] The substrate 10 is now coated by means of an epitaxial method.In this manner a layer 22 of epitaxial silicon first arises on thesurface of the porous layer 18. In principle, any of the known epitaxialmethods can be used for the formation of this layer 22, i.e. amongstother things gas phase epitaxy (CVD), ion assisted epitaxy, plasmaassisted epitaxy, liquid phase epitaxy and molecular beam epitaxy (MBE).

[0093] It can be seen from FIG. 1C that the free surface of the layer 22likewise has the same shape as the structured surface 12 of the siliconsubstrate of FIG. 1A and of the porous silicon layer 18 of FIG. 1B. Theboundary surface between the layer 22 and the porous layer 18 likewisehas the same shape. This applies, above all, if the porous layer 18 isthin. In this Figure the layer thickness is given by “w”.

[0094] Furthermore, the layer 22 has the same crystal orientation as thesubstrate 10 and the porous layer 18 formed from the substrate 10. Itconsists, moreover, of monocrystalline silicon.

[0095] In a further step, which is not especially shown here, a gridelectrode 24 is applied to the layer 22, and indeed in such a way thatthe material of the grid electrode 24 extends along only some of thelines forming the grid 16. Thereafter, the layer structure formed by thelayer 22 and the grid 24 is provided with a glass layer 26. This glasslayer 26 can be produced by the so-called Sol-Gel process, which is, forexample described in the publication “Sol-gel coatings for lighttrapping in crystalline thin film silicon solar cells” by R. Brendel, A.Gier, M. Mennig, H. Schmidt and J. H. Werner and was distributed at the“International Conference on Coatings on Glass ICCG” from Oct. 27 to 31, 1996 in Saarbrücken, Germany. In accordance with this, a mechanicalstress is produced in the porous layer, for example in that one “peelsoff” the glass cover disc 26 from the substrate 10, as shown in FIG. 1E.In this manner a separation of the layer-like structure from the porousstructure 18 takes place, with the layer-like structure consisting inthis example of the epitaxial silicon layer 22, the grid electrode 24and the glass cover 26. In this respect the separation takes place inadvantageous manner at the boundary surface between the porous layer 18and the epitaxial layer 22, with this boundary surface functioning, soto say, as a position of intended fracture, since here the mechanicalbond can be overcome most easily. Thereafter, in accordance with FIG.1F, the layer-like structure 28 is applied onto a metal plate 13 inaccordance with FIG. 1F and forms in this manner a solar cell. The metalplate 30 serves, on the one hand, for the provision of a contact to thepyramid-like tips 32 of the monocrystalline silicon layer 22 and serves,on the other hand, as a reflector, so that light, which has not yet beenabsorbed in the silicon, is sent through the layer 22 again, so that afurther absorption possibility is provided.

[0096] One notes that the grid 24 is filigrane and thus does not produceany notable light loss by reflection of the incident light 34.

[0097] The design of such a photocell will be explained in more detaillater with reference to the FIGS. 8 and 9.

[0098] As proof of the quality of the method, reference is first made tothe FIGS. 2, 3 and 4. Each of these Figures shows an electron microscoperecording, and indeed in FIG. 2 from the top side of an epitaxial Silayer 22 as shown in FIG. 1, in FIG. 3 from the free surface of theporous layer formed after the removal of the epitaxial Si layer 22. FIG.4 is a further recording of the epitaxial layer 22, but from a differentperspective) from which the problem-free profiling of the boundarysurface to the porous layer 18 can be seen.

[0099] In the recording of FIG. 3 one sees the surface of the substrate18 after the separation of the epitaxial layer 22, but before thecleaning of the free surface from the residues of the porous layer 18.After the cleaning, which can take place by means of etching and/or anultrasonic treatment, the free surface of the substrate 10 presentsitself in the same clean form as was present for the production of theporous layer 18 and the growth of the epitaxial silicon layer 22. Thus,the substrate 10 can be provided with a new porous layer 18 and reusedfor the growth of further layer-like structures or semiconductor layers,precisely as the layer 22 of FIG. 1C.

[0100] This is thus a first possibility of multiply using the substrate10.

[0101]FIG. 5 shows in schematic form a variant of the method of theinvention in which a multiple structure is produced.

[0102] In the example of FIG. 5 a substrate 10 is likewise present inthe form of p-or n-Si and a structured porous layer 18 is also presenthere above the substrate 10. The structuring of the initially freesurface 19 of the porous layer 18 corresponds, for example, precisely tothe profiling in the corresponding boundary surface in the embodiment ofFIG. 1. That is to say, the reference numeral 19 represents theuppermost boundary surface of the porous layer 18 (in other words theboundary surface of the porous layer 18 remote from the substrate 10).

[0103] Two sequential layers of n-Si and p-Si, i.e. the layers 22A and22B, are now grown by an epitaxial method onto the substrate 10, i.e.onto the structured surface of the porous layer 18. After the productionof the two layers 22A and 22B, the free surface of the p-Si layer 22Binitially extends up to the height 40 and has the same profiling as theboundary surface 19. Thereafter, the layer 22B is treated in order toform it into a further porous layer 18A in the upper region, which, forexample, corresponds in form to the porous layer 18 of FIG. 1. Themethod is now multiply repeated, whereby further layers 22A′, 22B′,22A″, 22B″, 22A′″, 22B′″ etc. arise, and each time the free surface ofthe upper layer 22B (B′, B″, B′″ etc.) is treated in order to produce aporous Si-layer 18A′, 18A″, 18A′″.

[0104] The multiple structure of FIG. 5 can then be split up in that oneseparates the individual layer-like structures 22A, 22B (in the sequence22A″″, 22B″″), (22A′″, 22B′″), . . . (22A, 22B)) from the multiplestructure. The splitting up of the layer packet in accordance with FIG.5 into individual structures, which each consist of an n-layer and ap-layer, i.e. 22A, 22B and 22A′, 22B′, and 22A″, 22B″ etc., can alsotake place in that one lays the layer packet for a relatively long time,for example in the range from hours to days in an etching bath. It isonly problematic that the useful layers 22A, 22B and 22A′, 22B′ etc.will also be etched at the surface, although the etching of the porousmaterial takes place substantially more rapidly.

[0105] In an alternative method of carrying out the separation, acarrier can in each case be connected to the free surface of the nextlayer pair to be separated, or the separation can, for example, takeplace through thermal gradients. After cleaning the layer pairs can thenbe provided with electrodes as desired.

[0106] It is apparent that the layer pairs 22A, 22B etc. explained inthe examples relating to FIG. 5 form an n-/p-junction. After theapplication of any electrodes that are required, the one surfaces of thelayer pairs can then be connected to a carrier material, for exampleprovided with a glass layer, as provided in FIG. 1. The possibility nowexists of turning over the so-treated layer pairs and applying furtherstructures by epitaxial methods onto the then free surface of therespective lower layer 22A (22A′, 22A″ etc.). Under some circumstancesthe substrate 10 with the structured porous layer 18 can be reused.

[0107] At this point it should be emphasised that the structuring of thefree surface of the porous Si layer is in no way restricted to thepreviously mentioned inverted pyramid shape. In actual fact, the mostdiverse structurings can be selected, as desired.

[0108] This is, for example, also brought to expression by theembodiment of FIG. 6. FIG. 6A namely shows again the Si-substrate 10with a porous Si layer 18. In this case the porous Si layer 18 has agroove-like profiling 50, consisting of longitudinal grooves 52 whichare arranged alongside one another, which are respectively separatedfrom one another by webs 54 of the porous silicon material. Thesegrooves 52, or the corresponding webs, can be made in accordance withany desired methods, for example also be mechanical milling, or by localcrushing of the porous layer 18 with a coining tool or a profiled roll.

[0109] It is now schematically illustrated in FIG. 6A how an adhesive 56is applied to the surface of the porous layer. This adhesive 56 servesfor the attachment of a second substrate 58, which can consist of anydesired material, onto the first substrate 10, so that the finishedstructure in accordance with FIG. 6B results. If a mechanical separatingprocess is now carried out, then the second substrate 58 together withthe adhesive layer 56 and the sections 54A of the webs 54 can beseparated from the first substrate 10 and the web remainders 54B, asshown in FIG. 6C.

[0110] As a result of the manufacturing method, the porous layer 18 hasthe same crystal orientation as the substrate 10 and this crystalorientation is accordingly also contained in the webs 54. Moreover, thiscrystal orientation is the same in all webs 54 and this also applies tothe sections 54A which are secured to the second substrate 58. Thesubstrate 58 with the sections 54A can now be used in order to growfurther structures onto the free surface having the sections 54A bymeans of epitaxial methods. In this way, a monocrystalline semiconductormaterial again arises on the substrate 58, i.e. monocrystalline siliconon the webs 54A.

[0111] The first substrate 10 can now be reused in that the remainder ofthe porous layer 18 is fully removed and the method of FIG. 6 repeated.The repetition of the method can take place multiply.

[0112] Even though, in the embodiment of FIGS. 6A to 6C, this structureis preferably realised using a profiled or structured surface, thepossibility also exists in accordance with the invention of operatingwith a non-structured porous layer, above all, but not exclusively, whenthe separation takes place with the method described in conjunction withFIG. 22 at the boundary surface to the porous substrate.

[0113]FIG. 7 shows a further embodiment of the method of the invention,likewise in a schematic representation.

[0114] Here, a cylinder of single crystal silicon is continuouslytreated in order to produce a porous silicon surface. For this purposethe lower segment of the cylindrical bar 60 is dipped into a HF bath anda voltage is produced between the grid electrodes 62 and the cylindricalbar 60 which leads to a current flow which, in combination with the HFbath, serves for the production of the porous Si layer.

[0115] During the rotation of the cylindrical bar 60 flexible substratematerial is applied onto the exposed surface of the porous Si layer, forexample sprayed on with subsequent curing, and is used in order to peeloff the porous Si-layer from the surface of the cylindrical bar 60.Since the porous Si layer 18 was originally curved, but now extends in astraight line due to the pulling off by means of the substrate 10, ithas a permanent strain which can be exploited for the production of somecomponents. This variant has the advantage that one produces astrip-like structure, i.e. a strip-like substrate 10 with a strip-likeporous layer 18 which can be used for the most diverse purposes.

[0116] For example, the porous layer 18 can be structured and then usedto carry out one of the previously explained methods. I.e. asemiconductor layer is initially produced by an epitaxial method on thefree surface of the porous Si layer 18, optionally after it has beenstructured, with the corresponding semiconductor layer or layers in turnconsisting of single crystal material.

[0117] If a flexible substrate is later made into a tube and thensubjected to epitaxy, a single crystal Si tube will arise. This could beof importance as a silane feedline for epitaxial reactors, because asilicon tube is mechanically very stable and does not contain anyforeign atoms. Also it could be used, as a result of the flexibility,for the manufacture of extended foils on optionally curved glasssurfaces, for example for vehicles operated with solar energy.

[0118] The different methods which are used will now be explained inmore detail,

[0119] A. Method of structuring the substrate wafer with invertedpyramids:

[0120] a) Oxidation (1% Trans LC) of the (100) orientated and polishedSi wafer at 1000° C. for 45 min. A 100 nm thick SiO₂ layer arises.

[0121] b) Photoresist spun on and photolithographically exposed using anet-like mask. As a result of the mask geometry, the photoresist onlyremains after development on ca. 2 μm broad webs, 11×11 μm² freesurfaces are present between the webs.

[0122] c) The oxide is removed in ca. 2 mm with buffered HF. Thephotoresist is removed.

[0123] d) RCA 1 and RCA 2 cleaning concluded with HF dip.

[0124] e) The inverted pyramids are etched in an 8% KOH solution at atemperature of 80° C. in 10 min. After the etching process, the sampleis rinsed in high purity water and dried. Oxide webs, to the extent theyare still present, are removed. With this anisotropic etching technique,crystal surfaces of the orientation (111) arise. The free bonds at the(111)-surface can be stably saturated with hydrogen, so that thecreation of SiO₂ at the surface is reliably avoided. Thus, for thesubsequent epitaxial steps, methods and reactors can also be consideredwhich do not permit a thermal decomposition of the oxide,

[0125] f) Alternative method:

[0126] f1) randomly arranged pyramids by anisotropic etching in KOH (nophotolithography)

[0127] f2) mechanical grinding with specially shaped saw blades (typicalstructure size 100 μm)

[0128] f3) porous silicon profiled in depth is produced by non-uniformillumination (n-type Si) which is then removed again

[0129] f4) the starting wafer can be multicrystalline Si, for exampleblock-cast material.

[0130] B. Method of manufacturing the porous layer at the surface of thestructured wafer:

[0131] a) The wafers are B-doped with an acceptor concentration between50×10¹⁸ cm⁻³ and 2×10¹⁹ cm⁻³. RCA1 and RCA2. Removal of the residualoxide with HF.

[0132] b) The etching apparatus corresponds to that laid open in patent0 536 788 A1, FIG. 2b. The porous silicon is produced by anodic etchingin HF:H₂O:Ethanol=1:1:2 at room temperature. The structured side of thesubstrate faces towards the cathode. The porosity of the layer isregulated by the current density, typical current densities amount to 1to 100 mA/cm².

[0133] c) We produces a ca.150 nm thick first porous layer of lowporosity (ca. 35%), followed by a ca. 10 μm thick second porous layer ofhigher porosity (50%).

[0134] d) The silicon disc with the structured and porous surface isoxidised at 400° C. for 30 min in dry O₂— atmosphere and stored underinert gas (N²) prior to epitaxy

[0135] C. The epitaxial method of ion-assisted deposition:

[0136] This process is described in detail in the paper “Crystallinethin film silicon solar cells by ion-assisted deposition” by S. Oelting,D. Martini, D. Bonnet. The samples from which SEM recordings were madein accordance with FIGS. 2 to 4 were subjected to epitaxy as follows:

[0137] a) RCA 1 and RCA 2 cleaning with an HF dip (30 s in 5% HF,subsequent rinsing in deionised water).

[0138] b) Introduction into the reactor and permitted to outgas at 400°C. to 500° C.

[0139] c) Heated to 850° C. for 10 min. for the removal of the residualoxide.

[0140] d) 10 μm thick silicon layers (measured parallel to themacroscopic surface normals of the substrate wafer) were grown withGa-doping of 5×10¹⁷ cm⁻³. The temperature of the Ga-crucible is 670° C.,the substrate temperature amounts to 700° C. The rate of deposition is 4μm/h. The coating takes place in high vacuum (<10⁻⁷ mbar). Layers withdoping sequences, in particular a pn-junction produced during epitaxywere likewise satisfactorily separated.

[0141] e) Alternative Epitaxial Method:

[0142] e1) liquid-phase epitaxy (LPE). Interesting because LPE ispossible at temperatures below 850° C.

[0143] e2) solid-phase crystallisation (SPC) of amorphous Si (a-Si).Interesting because deposition plants for large area a-Si-deposition areprior art. Disadvantage, SPC is slow (5 . . . 10 h for therecrystallisation)

[0144] e3) gas-phase epitaxy (CVD) as in patents by Canon. Disadvantage,CVD requires deposition temperatures >900° C. at which the porousmaterial sinters together. The mechanical separation is difficult orimpossible.

[0145] e4) plasma-assisted gas-phase epitaxy (LPCVD). Interestingbecause it is possible at low temperatures.

[0146] e5) hot-wire epitaxy, because higher deposition rates (>10Angström/S) possible at low temperature (<600° C).

[0147] e6) laser crystallisation of amorphous Si, because it is rapidand only involves low temperature loading of the substrate and of theporous Si.

[0148] D. Process for the separation:

[0149] a) The 10 μm thick epitaxial layer on the porous Si on thesubstrate wafer is laid onto a heating plate at a temperature of 125°.The Epi-layer upwardly.

[0150] b) Glycolphtalate is placed on the heated Epi-layer and a 2 cm×2cm=4 cm² sized cover glass is laid on this in turn. This transparentpolymer softens, flows under the weight of the glass plate and thusleads after 10 minutes to a full exclusion of air in the region betweenthe Epi-layer and glass. After cooling down, the glass is connected tothe structured Epi-layer.

[0151] b1) Using a different adhesive from glycolphtalate, for exampleplastics customary in the photovoltaic field.

[0152] b2) Use of mechanical carrier other than glass, for exampleplastic foils. Such flexible carriers can exploit the fact that a thinstructured Epi-layer is also very flexible (flexible solar cells).

[0153] b3) Use of Sol-Gel glasses which are cast onto the Epi-layer andthen harden. Particulars of Sol-Gel techniques are described in thesection “Experimental” of the paper “Sol-gel coatings for light trappingin crystalline thin films” presented on the occasion of the“International Conference on Coatings on Glass” which took place inSaarbrücken, Germany, from Oct. 27-31, 1996.

[0154] b4) Anodic bonding of the structured Epi-layer onto glass or“direct wafer bonding” of the epi-layer onto Si.

[0155] c) The glass with the epi-layer is now simply lifted off. Theporous layer is partly broken at the middle, it partly remains on thesubstrate and partly sticks to the epi-layer. A two-minute ultrasonictreatment removes all porous Si residues. The epi-layer bonds firmly tothe glass. Little mechanical force is required for the lifting of theglass with the epi-layer from the substrate when ultrasonic treatment iscarried out before the lifting.

[0156] Alternative methods for the mechanical separation:

[0157] c1) shock-like heating (for example) of the epi-layer with alight pulse produces a large temperature gradient in the porous layerwhich leads to the fracture of the porous layer.

[0158] C2) the fig of a liquid or of gas into the hollow cavities of theporous layer. The liquid or the gas is caused to expand and thus cracksoff the epi-layer.

[0159] c3) larger mechanical pressure on the epi-surface.

[0160] c4) resonant coupling of radiation into the porous layer whichfunctions as a wave guide and thus concentrates the radiation at theporous material.

[0161] Some semiconductor components will now be described which can berealised by means of the present invention.

[0162] The FIGS. 8 and 9 show first of all a photocell, here in the formof a solar cell—and comprises in the core a layer-like structure 22,consisting of a layer of n-type Si and having the same shape as thelayer 22 of FIG. 1.

[0163] The aluminium plate or foil 30 is located at the lower side ofthe layer-like structure 22 and is in contact with the pyramid-shapedtips 32 of the layer-like structure 22. Through a heat treatmentaluminium atoms diffuse into the tips of the layer-like structure 22,which is shown by the reference numeral 70, and produce there p-type Siinstead of n-type Si, i.e. the pn-junction, which is necessary for aphotocell, is provided in this manner.

[0164] As an alternative to this, the layer-like structure 22 could, forexample, consist, in accordance with FIG. 5, of a first layer 22A ofn-type Si and a second layer 22B of p-type Si, which is indicated by theboundary surface 22C shown in broken lines. The design of the lowerreflector, which simultaneously forms an electrode, remains the same aspreviously described.

[0165] Above the layer-like structure 22 there is located the gridelectrode 24, which in this example has the finger shape which can beseen from FIG. 9.

[0166] In the practical embodiment the fields are somewhat other than isshown in FIG. 9. Each finger 25 of the grid electrode has a width ofapproximately 20μ, i.e. approximately twice the width dimension of theindividual pyramids of the layer-like structure 22. Furthermore, gridfingers 25 are not present, as shown, at every fifth grid line, butrather a much larger number of uncovered grid cells lies between them,for example 1000.

[0167] It is also entirely possible to produce the grid electrode 25from a transparent material, for example indium tin oxide. The gridelectrode 25 can also be applied over the full area on the underside ofthe plate 26 or onto the top side of the layer-like structure 22.

[0168] The method of applying the glass plate takes place in the mannerwhich will be described later.

[0169] For solar applications the structuring in the Si layer isimportant because only in this way can so much sunlight be absorbed in athin layer. In distinction to known methods (direct silicon depositionon flat or textured glasses) both the front side and also the rear sideare freely accessible in the method described here.

[0170] Complicated contact schemes (such as are for example described inthe publication Appl. Phys. Letters, vol. 70, No. 3 of Jan. 20, 1997,pages 390 to 392) are not required. The solar cell manufacture isparticularly simple if one already produces the p-n-transition duringepitaxy, i.e. with the layers 22A and 22B and then clamps the layer-likestructure, i.e. the waffle, simply between a metal mirror (for examplethe above described sheet of aluminium 30) and a transparent conductor(for example indium tin oxide or zinc oxide) on a carrier material, forexample glass. Then no vapour deposition of contact fingers is anylonger required. The mechanical pressing is sufficient.

[0171] The reusability of the structured substrate wafer is an importantaspect for solar cell applications. It should be possible to reduce thethickness of the porous layer 18 from the previously used experimentalvalue of 10 μm to less than 1 μm. The smaller the porous layer can bemade, the more frequently the substrate wafer can be reused.

[0172] The FIGS. 18 and 19 show a schematic representation similar toFIGS. 8, 9, but in a modified embodiment. Here, the structuring of thelayer-like structure is effective somewhat differently, so that selectedpyramid tips 22D of the upper layer 22A point upwardly, i.e. they extendhigher than the other pyramid tips. This embodiment illustrates, how, bycunning choice of the structuring of the substrate, the frequency withwhich contact is established to the layer-like structure can becontrolled independently from the grid period P.

[0173]FIG. 20 shows how diverse solar cells, for example in accordancewith FIGS. 8 and 9, can be connected in series in order to form amodule. As shown here, springs 80 are used in order to electricallyconnect the upper and lower electrodes or conductors to one another. Thetriple voltage of a solar cell can be tapped off between the points Aand B.

[0174]FIGS. 10 and 11 show in schematic form a possible embodiment of aradiation detector. A structured Si-epi-layer 22 bonded onto a glasssubstrate 10 produces many chambers 72 which are closed and are filledwith a fixed quantity of gas. The so formed chambers are closed by anupper glass plate 26. If radiation strikes through the glass into therespective chamber 72, the gas heats up, expands and bends the membraneformed by the layer-like structure 22. This expansion can be detectedwith piezoelements 74. If different regions of the detector are used fordifferent wavelengths of the radiation to be detected, then filters canbe provided, for example, in the upper glass plate 26, which onlytransmit the respective radiation to be detected.

[0175] In FIGS. 10 and 11 only four chambers 72 are shown. In practiceit will be more.

[0176] The structure of FIGS. 10 and 11 can also be used as a pressuresensor. The structured Si epi-layer, bonded onto glass, shows manyindividually closed chambers, which are filled with a fixed quantity ofgas. If the external pressure changes (air pressure or mechanicalpressure) then the chamber walls bend. Such bending can be detected ateach chamber individually with a piezoelement.

[0177] A further possible application of layer-like structuredstructures lies in the provision of special mirrors (micromirrors withspecial characteristics) which can be produced by a special structuringof the reflecting surface of the layer-like structure.

[0178]FIGS. 12A to D show a structure similar to that of FIG. 6, butwithout effecting a special profiling of the porous layer.

[0179] Specifically FIGS. 12A to D show a method of manufacturing asubstrate, onto which a single crystalline semiconductor layer can beapplied by means of epitaxy.

[0180] As a first step, a substrate 10 of a semiconductor material,preferably silicon, is treated in order to produce a porous layer inplate shape with planar boundary surfaces.

[0181] The adhesive 56—possibly already with carrier 58—is then broughtonto the porous layer so that the adhesive at least partly penetratesthrough the porous layer 18. Thereafter, the mechanical separation ofthe adhesive from the substrate takes place. If the adhesive is acompound which is sufficiently mechanically strong, then a carrier canbe dispensed with. That is to say, the adhesive itself forms thecarrier. The adhesive can, however, be reinforced with the carrier 58,if necessary.

[0182] The separation of the adhesive, possibly with carrier, from thesubstrate 10 takes place in such a way that the surface formed by theseparation is permeated with porous semiconductor material of thedesired orientation. The adhesive with this covering of porous material,and possibly with a carrier 58 at the side remote from the porousmaterial, then forms a substrate for the carrying out of later epitaxialmethods.

[0183] The substrate 10, which will normally have residues of porousmaterial, is first cleaned in order to remove these residues. Then a newporous layer is formed, so that the substrate 10 can be reused.

[0184] The FIGS. 13A to H show a method for the manufacture of asemiconductor layer which is monocrystalline in certain regions andamorphous in other regions.

[0185] In accordance with FIG. 13A a flat substrate is initiallypresent, which can consist of monocrystalline or polycrystallinesemiconductor material, for example Si.

[0186] The FIG. 13B shows in very schematic form that the one surface ofthe substrate is structured by the introduction of grooves or holes orof a desired pattern by grinding or etching, and indeed with a structuredepth h.

[0187] In accordance with FIG. 13C a porous layer of the thickness WPS≧his produced in known manner, for example by anodic etching in HF.

[0188] Thereafter, an adhesive, for example Sol-gel glass, is appliedonto the structured surface of the substrate and penetrates wholly orpartly into the porous layer. It forms a porous layer 18 permeated bythe adhesive, as is shown in FIG. 13D).

[0189] Thereafter, the mechanical separation of the adhesive from thesubstrate takes place in accordance with FIG. 13E, the part of theporous layer permeated with adhesive 56 bonding to the adhesive. Thesubstrate 10 can then be used further after a suitable surface treatment(removal of the residues of the porous layer and optionally newstructuring).

[0190] The second substrate consisting of the adhesive (possibly withcarrier) and of porous working material permeated by adhesive istreated, for example polished, in order to provide a layer-likestructure which contains porous material in some regions, and indeed ina well defined crystal orientation, but which has no porous material inother regions, as is shown in FIG. 13F.

[0191] Thereafter, in accordance with FIG. 13G, a full area depositionof an amorphous layer 76 onto the surface 78 provided in the step 13Ftakes place.

[0192] Thereafter, in accordance with FIG. 13H, a thermal treatment is,for example, carried out, so that a solid phase crystallisation of theamorphous material takes place, where the porous material embedded inthe adhesive provides nucleation seeds of well defined orientation, Thematerial remains amorphous at places where no porous layer is stored.The corresponding positions are positions where, in accordance with FIG.13B, recesses 14 were formed during the structuring of the substrate 10.The structure in accordance with FIG. 13H now forms the starting pointfor the manufacture of the product, such as a flat screen. It is namelypossible to so structure the product of FIG. 13H that luminescence isproduced in the amorphous regions, while control transistors are formedin the monocrystalline regions, which control the state of luminescencein the amorphous region.

[0193] A further interesting possibility in accordance with theinvention lies in first making a substrate porous close to the surface,as described above, with a part of the porous layer being converted intoa single crystal nonporous layer in deviation from the previousdescription, and indeed by rapid melting and subsequent solidification,instead of applying a crystalline Si-layer onto the porous layer bymeans of epitaxy. That is to say, the uppermost layer of the cavityexhibiting or porous layer is first fused at least locally and thensolidified again.

[0194] This can also be understood as a type of epitaxy on a poroussubstrate. However, the material for the epitaxy originates from theporous layer itself. After the production of the single crystalnon-porous layer by fusing and subsequent solidification of the porouslayer, the solidified layer can either be at once separated from thesubstrate or a layer-like structure can be grown onto the solidifiedlayer, and the solidified layer subsequently separated from thesubstrate.

[0195] As previously the separation takes place using the layer havingthe hollow cavities or porous layer as a position of intended fractureby the production of a mechanical strain within the cavity exhibiting orporous layer, or at a boundary surface of the cavity exhibiting orporous layer, or using the method described in conjunction with FIG. 22.

[0196] The fusing preferably takes place by irradiation with a laserlight pulse from an excimer or copper vapour laser. This can, forexample, take place in accordance with the method which is described inthe publication “Ultra-large grain growth of Si films on glassysubstrate” by Ishihara and M. Matsumura in Electronics Letters, Oct. 26,1995, Vol. 31, No. 22, pages 1956 to 1957.

[0197] In distinction to the method described in this publication,porous silicon is to be transformed here into single crystal Si. A shortlight pulse is of advantage in comparison to a long-term irradiation,which is likewise possible, because in this way one can fuse only theregion close to the surface and does not change lower lying porousmaterial. A technical problem could consist in the fact that the thermalgradients which arise lead to a splitting off of the crystalline layer.This can either be avoided by suitable conditioning of the porous Si, orone could carry out the layer manufacture and the separation in onestep, which is possible in accordance with the invention.

[0198] As an alternative to the laser treatment, zone drawing can alsobe considered as a method of rapid heating. In this the porous layer isguided beneath a linearly bundled electron beam or light beam, so thatan areal crystalline layer arises. A corresponding method can be seenfrom the publication with the title “A new fabrication method formulticrystalline silicon layers on graphite substrates suited forlow-cost thin film solar cells” in Solar Energy Materials and SolarCells 41/42 (1996), page 119 to 126 by M. Pauli, T. Reindl, W. Krühler,F. Homberg and J Müller, with this paper having been published byElsevier Science B. V.

[0199] The invention will now be described from another point of view.

[0200] In the following the process of perforated silicon (Ψ-process)for the manufacture of ultrathin silicon layers with efficient lighttrapping will be explained. For this a silicon layer is grownepitaxially on the porous surface of a structured, monocrystallinesilicon substrate. Mechanical stress breaks the porous layer and therebyseparates the epitaxial layer from the substrate. According to X-raydiffraction analysis, the W_(f)=5.8 μm thick silicon layer ismonocrystalline. Reflectance measurements and ray tracing simulationspredict a maximum short circuit current of j_(sc)*=36.5 mA/cm² forlayers in the form of a waffle, when they are secured to glass.Transport simulations predict an efficiency η=16 to 19% for a filmthickness of W_(f)=2 to 3 μm.

[0201] 1. Introduction

[0202] Thin layer solar cells of crystalline silicon are known from theliterature, for example [1]. This and subsequently named documents arenumbered within square brackets and set forth for the purpose of easierdigest at the end of the description in a list. Thin film solar cells ofcrystalline silicon pose essentially three requirements:

[0203] (i) the growth of a crystalline silicon layer of high quality andlarge grain size on an inexpensive substrate, (ii) the realisation of alight trapping scheme for the compensation of the intrinsically weak,near infrared absorption in crystalline silicon, and (iii) an effectivepassivation of the grain boundaries and surfaces.

[0204] A structured, monocrystalline silicon layer on a float glasswould contribute to the satisfying of all three requirements: (i)monocrystalline material can have a high volume quality and float glassis an inexpensive substrate; (ii) innovative layer structures [2 to 4],such as for example the pyramidal layer structure [4] permit theefficient trapping of light; (iii) the monocrystalline structureprevents grain boundary recombination and enables an effective surfacepassivation at low temperatures [5]. Such a fabrication of thin andstructured monocrystalline silicon layers has hitherto not been shown inthe literature.

[0205] In the following the novel process of perforated silicon for themanufacture of structured monocrystalline thin layers on float glasswill be explained. In this respect the light trapping behaviour of suchlayers is experimentally investigated. The possible efficiency of thenovel layer structure will be analysed theoretically.

[0206] 2. Process of the Perforated Silicon

[0207] The epitaxy on porous silicon was investigated in detail for themanufacture of thin monocrystalline silicon layers on isolatingsubstrates [6]. In this process an epitaxial layer grows by a CVDprocess at temperatures T>1000° C. on a planar, monocrystalline siliconwafer with a porous surface. The epitaxial layer is then applied bywafer bonding onto an isolator. Mechanical grinding then removes thesubstrate wafer. Chemical etching of the remaining porous layercompletes the process. The absence of light trapping characteristics,the bonding process and the consumption of the substrate wafer preventthe use of this technique for photovoltaics for cost reasons.

[0208] In contrast to this, the process presented in the following canbe used in photovoltaics, because the process facilitates the trappingof light, avoids bonding processes and no longer consumes the substratewafer. FIGS. 1A to F illustrate the process step for step, whichproduces a structured monocrystalline silicon layer on glass:

[0209] a) A monocrystalline silicon substrate wafer receives a surfacestructure by any type of etching or mechanical grinding. In this respectstructures are possible which are much more complicated than the regularinverted pyramids of the periodicity p in FIG. 1A.

[0210] b) The surface of the substrate is converted into a poroussilicon layer (porous Si-layer, PSL) of the thickness W_(PS). Theorientation of the silicon in the PSL passes on the informationconcerning the substrate orientation.

[0211] c) Silicon is subsequently grown epitaxially onto the PSL. A lowtemperature epitaxial technique is of advantage, because the surfacemobility of the silicon atoms at the inner surface of the PSL leads to asintering process at temperatures above 850° C. [7].

[0212] At this point in time the outer surface of the epitaxial layer isfreely accessible. Every process which works at temperatures below about850° C. can be used in order to form the emitter of the cell. Both anepitaxial emitter as well as an inversion layer or a heterojunctionemitter are possible. For surface passivation and grid formationinnovative techniques should be used, such as are described in [5, 8,9].

[0213] d) An overlying substrate (for example glass) is secured with atransparent adhesive to the front surface. The temperature stability ofthe overlying substrate and of the adhesive determine the maximumprocess temperature of all subsequent process steps.

[0214] e) The low mechanical strength of the PSL in comparison to thesubstrate silicon is exploited in order to separate the cell from thesubstrate. A plurality of ways of proceeding is possible: shock heating,filling of the holes with liquids or gases which are caused to expand,straining of the PSL by compressive or tensile stress, or ultrasonictreatment. In all these cases the PSL functions as a perforation insilicon (Psi) hence the name φ.

[0215] f) the rear side of the cell is accessible for surfacepassivation and the formation of a reflector. An offset reflector canalso serve to form point contacts, which are compatible with a lowrecombination of the minority charge carriers.

[0216] The free accessibility to the rear and front surface is anintrinsic advantage of the φ-process over processes which depositsilicon directly onto an insulating substrate.

[0217] The formation of the PSL consumes a thickness W_(PS)/Cos (α) ofthe substrate wafer which is so structured that the crystal surfacesstand an angle α to the macroscopic cell surface. After the removal ofthe total remaining porous silicon, the substrate retains the originalsurface morphology (FIG. 1A) providing W_(ps)/p<<1. Otherwise the edgesand tips are rounded with a gradius of curvature W_(PS)/p as shown inFIG. 1E. Thus, the substrate can be reused several times for adequatelysmall ratios W_(PS)/p until a new structuring of the substrate waferbecomes necessary.

[0218] 3. Experimental Investigations

[0219] 3.1 Preparation of the Sample

[0220] A monocrystalline silicon wafer of the p+-type doped with boronto 10¹⁹ cm⁻³, orientated in the (100)-direction and of four inchdiameter is given a structure of inverted pyramids with a periodicityp=13 μm by photolithography and anisotropic etching with KOH. Anodicetching in diluted HF produces a W_(PS)=6 μm thick porous silicon layerin a time of approximately 2 minutes. Prior to the epitaxy, the sampleis heated for 10 minutes to ca. 850° C. in order to remove the naturallyarising oxide from the PSL surface. An epitaxial Ga-doped silicon layerof a thickness of W_(PS)=5,8 μm is grown by the ion-assisted depositiontechnique ((IAD) [10] at 700° C. The growth rate amounts to 4 μm/h onplanar surfaces. Transparent poly-(ethylene-phtalate) secures glasssurfaces of the size 2×2 cm² to the epitaxial layer. An ultrasonictreatment of approximately 2 min. destabilises the PLS layer andfacilitates the mechanical removal of the epitaxial layer withoutchemical etching. It is also possible to separate the epitaxial layerand the substrate from one another without the ultrasonic treatment.

[0221] 3.2 Characterisation of the Sample

[0222]FIGS. 2 and 4 show scanning electron microscope recordings of afree standing silicon waffle structure which was produced with theφ-process. Apart from the ultrasonic treatment no farther cleaning waseffected before the scanning electron microscope investigations. Theperspective plan view of FIG. 2 shows regular inverted pyramids whichare copies of the original surface structure of the substrate wafer.FIG. 4 shows in oblique view the cross-section of the waffle structure.The pyramid tips point downwardly. No cracks can be seen. The layerthickness perpendicular to the pyramid-shaped crystal surfaces amountsto W_(f)=5.8 μm. The topside has non-visible recesses in FIGS. 2 and 4,the depth and diameter of which respectively amount to less than 0.1 μm,whereby a type of micro roughness is present. These recesses areassociated with the IAD-technique, since they also occur with flatepitaxial layers which are grown on non-structured substrate silicon.

[0223] Hall measurements on a layer, which was deposited on anon-structured monocrystalline substrate of high specific resistanceshow a concentration of the electrically active dopant material Ga of2×10¹⁷ cm⁻³ and a hole mobility of 186 cm²/Vs.

[0224]FIG. 14 shows the Cu_(Kα)X-ray diffraction spectrum of the siliconwaffle on glass in comparison to the spectrum of the monocrystallinesilicon substrate. The intensity is shown in the logarithmic scale. Allpeaks occur at the same angle. Thus, the silicon waffle structure ismonocrystalline and has the same orientation as the substrate wafer.Only the large (400) peak originates from silicon. All other peaks aremore than 2 orders of magnitude smaller and are artefacts of the X-rayapparatus. The high background intensity of the epitaxial layer iscaused by the amorphous glass substrate. Consequently, the IAD technique[10] enables the epitaxial growth on porous substrates.

[0225] The lifetime of the substrate minority charge carriers is one ofthe critical material parameters of a solar cell. The surface must bewell passivated in order to measure the substrate lifetime. Accordingly,a free standing silicon waffle is oxidised at 1000° C. on both sides.The surfaces are charged by a corona discharge chamber [11], in order torepel the minority charge carriers from the recombination centres at thesurface.

[0226]FIG. 15 shows the plot of the microwave reflectance afterexcitation with an optical pulse of 20 ns. The sample is arranged byquarter microwave wavelength above a metal reflector in order to achieveoptimum sensitivity [12]. The drop-off is not strictly monoexponential.It does, however, permit the lifetime to be estimated at τ=0.27 μs±0.08μs. The slow drop-off for times t>0.6 μs is caused by softening(de-trapping) of charge carriers in flat levels. The electron mobilitywas not measured. However, having regard to the measured hole mobilityμ=186 cm²/Vs, a minority charge carrier diffusion length L>11 μm iscalculated as the lower limit for the electron mobility and this isgreater than the film thickness W_(f)=5.8 μm.

[0227] For thin film cells the light trapping is important.Unfortunately, the ideal behaviour of the adhesively bonded wafflestructure which is provided with an aluminum mirror behind the sample asschematically shown in FIG. 1F cannot be measured without contacting thesample. Accordingly, the short current potential of the sample wasestimated from a comparison with a measured hemispherical reflectanceand a ray following simulation with the program SUNRAYS [13]. It wasshown that the offset reflector substantially reduces the optical lossesin the Al [2].

[0228]FIG. 16 shows the measured (continuous line) and the calculated(circle) hemispherical reflectance. The ray tracing simulationreproduces approximately the measurement without adaptation of theoptical parameters. Small deviations between the measurement and thesimulation are qualitatively explained by the microroughness of thepyramidal crystal surfaces, which were not taken into account in thesimulation [2]. SUNRAYS calculates a maximum short circuit currentj_(sc)*=36.5 mA/cm²±0.5 mA/cm² from the simulated absorption (triangles)for the W_(f)=5.8 μm thick waffle with a structure period p=13 μm on theradiation with an AM 1.5G spectrum of 1000 W/m². The error bars resultfrom the statistics of the Monte-Carlo-simulation.

[0229] 4. Possible Efficiency

[0230] The possible efficiency of crystalline silicon layers with theform shown in FIGS. 2 and 4 is investigated by theoretical modelling.The optical model uses ray tracing by SUNRAYS, as described above. Therate of the minority charge carrier production is set to be spatiallyhomogenous in the silicon layer and calculated from j_(sc)* and the cellvolume. In addition to the optical model, a model for the electrontransport is necessary. The complex three-dimensional charge carrierdiffusion in the silicon waffle is approximated by a purelyone-dimensional transport perpendicular to the pyramidal crystalsurfaces. The efficiency of the cell depends on the minority chargecarrier diffusion length L and the (surface recombination velocity, SRV)S. It is very important to optimise the cell thickness W_(f) in order tocorrectly estimate the possible efficiency for fixed L and S.[14].Accordingly, the simulation varies the film thickness W for an idealcell efficiency. A silicon cell is assumed with an emitter which isp-doped to 10¹⁹cm⁻³ and 0.5 μm thick and has a base which is B-doped10¹⁸ cm⁻³. For thick W<1 μm the base and the emitter are of the samethickness. The diffusion length L and the surface recombination speed Sare equated for the base and the emitter in order to reduce the numberof free parameters. The recombination in the space charge zone isexplained in [15]. The mobility values and parameters of the bandgapnarrowing of c-silicon are taken from the document [14].

[0231]FIG. 17 shows the efficiency (continuous line) for optimum cellthickness (broken line) for a broad range of the parameters S and L.With a diffusion length L=11 μm, an energy conversion efficiency of 16to 19% is calculated in dependence on the surface recombination speed Sfor an ideal cell thickness of 2 to 3 μm (points). An efficiency of 16%corresponding to an SRV S=10⁴ cm/s would be a great success for 2 μmthink crystal silicon solar cell on glass. The deposition of a W_(f)=2μm thin layer requires 50 minutes with the currently used IAD technique.

[0232] The new process of perforated silicon (p-process) has beenexplained. The epitaxy on a structured monocrystalline silicon substrateand the mechanical separation of the epitaxial layer from the substrateresult in ultrathin monocrystalline structured silicon layers on anytype of glass. Measurements of reflection capability shown an opticalabsorption which corresponds to a maximum short circuit currentj_(sc)*=36.5 mA/ cm². Theoretically, the material quality is sufficientfor an efficiency of 16 to 19% at an ideal cell thickness which extendsfrom W_(f)=2 to 3 μm.

[0233] Further possibilities of the φ-process lie in the small thicknessWPS<1 μm of the porous layer in order to reduce the material consumptionand the enabling of frequent reusability of the substrate wafer. Afurther increase of the deposition rate is likewise possible. Ultrathinlayers of 100 cm² size can be produced without problem.

[0234] A further, specially preferred embodiment of the invention isdescribed in the following and is concerned with the manufacture of asolar cell. The method described here is not restricted to a photocell,but should rather be understood as a general manufacturing process. Themotivation for this embodiment lies in the fact that a series connectionof solar cells permits power extraction from the solar cell model athigh voltages and small currents. Small currents reduce ohmic losses.The contact fingers on the solar cell front and rear side also have thepurpose of reducing ohmic losses. With a suitable integrated seriesconnection, the contact tracks can therefore by avoided.

[0235] The realisation of such a series connection takes place throughthe use of shadow masks. Here, a series connection of the solar cells isalready effected during the layer manufacture, i.e. the layers areselectively grown (at certain locations, but not at others). Theposition of the layer growth is controlled by the shadow mask. Theshadow mask is preferably realised by tensioned wires.

[0236] An example of a series connection using the (p-process will bedescribed in the following with reference to FIG. 21. One possibility ofthe layer deposition of porous material is the ion-assisted depositiontechnique (IAD). In the IAD technique the transport of the silicon atomsis very directed, as in the vapour deposition technique. This isexploited in accordance with the invention in order to produce anintegrated series connection during the layer manufacture through theuse of shadow masks.

[0237]FIG. 21 shows the simple process sequence in a schematicillustration. The textured porous substrate 18, i.e. a structuredsubstrate, which can be manufactured as previously described is shownfor the sake of simplicity without texture. If the shadow mask 301 islocated in the position 1, then individual regions 300 of thep+-Si-layer grow which are separated from one another by a trench 302.Thereafter, the same mask is shifted by a certain amount horizontallyand individual regions of the p-Si-layer 304 grow. In the third positionindividual regions of the n+-Si-layer 306 are finally produced whichthen complete the series circuit by overlapping the exposed regions ofthe first p+-Si-layer. The definition of the surface areas of theindividual layers in the direction transverse to the trenches of theindividual layers 300, 304, 306, for example to the trenches 302, can beensured by a further shadow mask, which does not have to be shifted, butwhich can optionally be shifted.

[0238] The advantages of this type of series connection are:

[0239] The application of metal contacts is omitted. The etching of thetrenches is avoided. Both save process costs.

[0240] Only one mask is required, the accuracy of its shape isuncritical.

[0241] The application of wire grids is avoided.

[0242] If the wires are fixed outside of the hot region of the reactor,the wire spacing does not change during heating up. This avoids theknown problems with the thermal expansion of large masks.

[0243] Shadow masks are coated during deposition and become unusable inthe course of time. A wire mask can be easily renewed by rereeling wiresin the reactor.

[0244] The most diverse modification are possible, for example:

[0245] 1. The sequence of the dopings can be exchanged, i.e. n+ at thebottom, p+ at the top.

[0246] 2. Further layers come onto the emitter, such as for examplemetal layers or semitransparent metal oxide or layer systems ofconductive and nonconductive layers to enhance the cross-conductivity ofthe emitter, all of which can be applied by shadow masks.

[0247] 3. The wire diameter and the wire spacing can be varied fromlayer to layer.

[0248] 4. The relative position of the wire mask and the sample, or thesubstrate, is continuously changed during the deposition of a layer.

[0249] 5. The shadow mask consists not only of wires but rather of metalstrips.

[0250] 6. The principle of the shadow mask can also be used in order toproduce semiconductor layers of any desired external shapes. If, forexample, a mask with a circular opening is held in front of the porousSi-layer, then a round single crystal semiconductor layer arises whichcan be used as a solar cell in watches. A subsequent cutting of thesemiconductor layer into the desired shape is avoided.

[0251] As mentioned in the introduction to the specification it can bedifficult in the mass manufacture of electronic components using porouslayers to carry out the mechanical separation as cleanly as is possiblein the laboratory. This is to be attributed to the fact that the poroussilicon has two functions. On the one hand, it permits the epitaxial,i.e. well-orientated growth of a non-porous layer. The lower theporosity, the better this first function is satisfied On the other hand,the silicon functions as a position of intended fracture for themechanical separation. The higher the porosity, the better this secondfunction is satisfied. A good quality and simultaneously releasableepitaxial layer therefore requires a balanced compromise between higherand lower porosity. Our experiments show that this compromise isfrequently difficult to realise. Thus we have frequently observed thatthe epitaxial layers separate undesirably and uncontrollably during thesubsequent epitaxial process. The methodology of EP-A-0 767 486 attemptsto overcome this problem in that ions are implanted into the porouslayer at a specific level or by changing the current density during theanodising for the manufacture of the porous layer. In both cases,regions of the porous layer with higher porosity arise at which themechanical separation preferentially takes place. These method variantsare, however, relatively complicated and disadvantageous.

[0252] The ion implantation represents a further process step. Theenhanced porosity in accordance with both method variants can lead to afracture or to separation of the substrate at an undesired point intime, i.e. for example during the subsequent epitaxial process.

[0253] The following method variant described in conjunction with FIGS.22A to 22E provides assistance here:

[0254] The method comprises the following steps:

[0255] 1) A reusable silicon substrate wafer is made porous wholly orclose to the surface. On using an n-type wafer an additionalillumination is necessary to make it porous by anodic etching. In thefollowing, we assume the substrate wafer has been made fully porous. Theporosity is low (greater than 0% and smaller than 50%, preferably 10 to20%) in order to give the substrate wafer a high mechanical stability.

[0256] 2) The mechanically stable low porosity substrate wafer 18 inFIG. 22A is shown here for the sake of simplicity with a planar surface,although a structured surface, as for example in FIG. 1, is preferred.Epitaxy is carried out on this substrate wafer 18 on the texturised orflat surface (for example an n-type Si-layer 400 is formed on it FIG.22B). The porous substrate endows the thin epitaxial layer withmechanical stability which permits a solar cell process to be carriedout without the danger of undesired separation. The solar cell processcan also include the deposition of a p-type epitaxial layer 402 onto then-type epitaxial layer 400, as is for example shown in FIG. 22C.

[0257] 3) The solar cell is adhesively bonded onto a possiblytransparent carrier 404, as shown in Fig, 22D. The porous substrate andthe epitaxial layer or the solar cell which are fixedly connectedtogether now bond onto the carrier. A mechanical separation from theporous substrate and the epitaxial layer is not possible because of thelow porosity.

[0258] 4) For the separation of the porous substrate from the epitaxiallayer and the carrier, the total structure is again dipped into anetching solution containing HF. The solution penetrates through theporous substrate without etching it, because adequate holes must be madeavailable for the etching process. Holes are now brought by means ofillumination through the transparent carrier or by the application of anadequately large voltage between the electrolyte and the epitaxial layerto the boundary surface between the porous substrate and the epitaxiallayer. If the hole concentration is sufficiently large, the boundaryregion is either made mechanically so unstable by producing furtherporosity that the epitaxial layer with the carrier can subsequently beseparated from the substrate by mechanical loading in the sense of claim1, or the silicon is completely dissolved by a particularly high holeconcentration (electro polishing) so that the epitaxial layers 400, 402and the carrier separate from the porous substrate 18 (FIG. 22E). Thecarrier 404 with the two epitaxial layers, which preferably have thestructuring in accordance with FIG. 8, or FIG. 18, or FIG. 20 or FIG.21, can be completed via a reflector into a solar cell or can be treatedfurther for the production of other components.

[0259] A high doping of the porous substrate wafer, for example a dopingdensity in the range from 10¹⁸ to 10¹⁹ cm⁻³, and also the large surfacerecombination in porous Si guarantee that the hole concentration hasalready dropped off by recombination in a depth of the porous substrateof 0.1 to 10 μm that the greatest part of the porous substrate remainspreserved and can be reused. In the event of a p-type epitaxial layer ona p-type porous substrate, the separation technique described hereexploits the finding known per se that the current density selectedduring anodic etching only influences the porosity of the porousmaterial newly arising at the boundary surface. If this porosity isselected close to 100%, then the epitaxial layer separates and thelargest part of the porous substrate remains for reuse.

LIST OF LITERATURE

[0260] [1] J. H. Werner, R. Bergmann, and R. Brendel in“Festköperprobleme/Advances in Solid State Physics”, Vol. 34,herausgegeben von R. Helbig (Viewg, Brauschweig, 1994), Seite 115

[0261] [2] R. Brerdel, in “Proc. 13 ^(th) European Photovoltaic SolarEnergy Conf.“, Her. W. Fretesleben, W. Freieseben, W. Palz, H. A.Ossenbrink, and P. Helm, (Stephens, Bedford, 1995), Seite 4365

[0262] [3] D. Thorp, P. Campbell and S. R. Wenham, “Progress inPhotovoltaics 4”, 205 (1996)

[0263] [4] P. Brendel, R. B. Bergmann, P. Lölgen, M. Wolf and J. H.Werner, “Appl. Phys. Lett. 70”, 390 (1997)

[0264] [5] T. Lauinger, J. Schmid, A. G. Aberle and R. Hezel, “Appl.Phys. Lett. 68”, 1232 (1996)

[0265] [5] N. Sato, K. Sakguchi, K. Yamagata, Y. Fujiyama and T.Yonchara, “J. Elecrochem. Soc. 142”, 3116 (1995)

[0266] [7] C. Oules, A. Halimaoui, J. L. Regolini, R. Herino, A. Perio,D. Bensahel and G. Bomchil, “Materials Science and EngineeringB4”, 435(1989)

[0267] [8] R. Hezel, in “Proc. 24^(th) IEEE Photovoltaic Specialistsconf.”, (IEEE, New York, 1995), Seite 1466

[0268] [9] G. Willeke and P. Fath, “Appl. Phys. Lett. 64”, 1274 (1994)

[0269] [10] S. Oelting, D. Martini and D. Bonnet, in “Proc. 12^(th)European Photovoltaic Solar Energy Conf.”, herausgegeben von R. Hill, W.Palz and P. Helm, (H. S. Stephens, Bedford, 1994), Seite 1815

[0270] [11] M. Sch{overscore (o)}fthaler and R. Brebdel, in “Proc.1^(st) World Conf. Photocoltaic Energy Conversion, (IEEE, New York,1994), Seite 1509

[0271] [12] M. ScH{overscore (o)}fthaler and R. Brendel, “J. Appl. Phys.77”, 3162 (1995)

[0272] [13] R. Brendel, “Progress in Photovoltaics 3”, 25 (1995)

[0273] [14] M. J. Stocks, A. Cuevas and A. W. Blakers, “Progress inPhotovoltaics 4”, 35 (1996)

[0274] [15] S. C. Choo, “Solid-St. Election 39”, 308 (1996), Eq. 3

1. Method of manufacturing layer-like structures in which a materiallayer having hollow cavities, preferably a porous material layer, isproduced on a substrate consisting, for example, of monocrystallinep-type or n-type Si and wherein the layer-like structure, or a part ofit, is subsequently provided on the cavity exhibiting or porous materiallayer and is subsequently separated from the substrate using the cavityexhibiting or porous layer as a position of intended separation, forexample through the generation of a mechanical strain within or at aboundary surface of the cavity exhibiting or porous layer characterisedin that the surface of the substrate is structured prior to thegeneration of the porous layer, or in that the surface of the porouslayer is structured.
 2. Method in accordance with claim 1, characterisedin that the surface of the substrate is structured by one or more of thefollowing methods: a) by a photolithographic method, b) by an etchtreatment, for example by a treatment of n- or p-silicon with KOH toproduce random pyramids at the surface of the substrate, c) by achemical method, d) by mechanical milling, e) by laser treatment. 3.Method in accordance with claim 1, characterised in that the surface ofthe porous layer is structured by one or more of the following methods:a) by a photolithographic method, b) by an etch treatment, for exampleby a treatment of n- or p-silicon with KOH to produce random pyramids atthe surface of the substrate, c) by a chemical method, d) by mechanicalmilling, e) by laser treatment f) by mechanical coining.
 4. Method inaccordance with one of the preceding claims, characterised in that thelayer-like structure is applied onto the porous surface at least partlyby an epitaxial method (homo-epitaxy or hetero-epitaxy) with at leastone semiconductor layer belonging to the layer-like structure beingapplied onto the surface of the porous layer by the epitaxial method. 5.Method in accordance with one of the preceding claims, characterised inthat the layer-like structure is formed at least partly by theapplication or deposition of a metal layer, for example in the form ofan aluminium foil or of an aluminium sheet, which is applied to theadjacent material of the layer-like structure by heating and surfacediffusion.
 6. Method in accordance with one of the preceding claims,characterised in that the step of forming the layer-like structureincludes the application of a dielectric, for example in the form of atransparent or light-permeable window layer, for example by the Sol-gelprocess, or by means of an adhesive.
 7. Method in accordance with one ofthe preceding claims, characterised in that a carrier layer is providedwhich is either brought into connection with the layer-like structure,for example by adhesive bonding, by wafer bonding, or by a diffusionbrazing method, or is formed as a part of the layer-like structure. 8.Method in accordance with one of the preceding claims, characterised inthat, after the separation of the layer-like structure from thesubstrate, a further structure is produced on or applied to theoptionally structured surface of the layer-like structure forming theposition of intended fracture.
 9. Method in accordance with claim 8,characterised in that the surface formed by the position of intendedfracture is cleaned and/or partly removed and/or newly structured ormade porous prior to the production of the further structure.
 10. Methodin accordance with one of the preceding claims, characterised in that,after the separation of the layer-like structure from the substrate atthe position of intended fracture that is provided, the substrate isused again, with or without the remainder of the porous layer, as asubstrate for the application of a layer-like structure
 11. Method inaccordance with claim 10, characterised in that, with renewed use of thesubstrate having a structured porous layer, i.e. a porous layer having anon-plane parallel plate form, the latter is subjected to a cleaningstep carried out, for example, by etching or by an ultrasonic cleaningprocess.
 12. Method in accordance with one of the preceding claims,characterised in that a further porous layer is produced on the surfaceof the layer-like structure remote from the substrate prior to or afterseparation of the layer-like structure from the substrate, and a furtherlayer-like structure is formed hereon, with the method optionally beingrepeated a plurality of times, whereby a plurality of layer-likestructures, in particular structured layer-like structures, arise aboveone another, which are respectively separated from the adjacentlayer-like structure by a porous layer forming a position of intendedfracture, wherein, after producing such a multiple structure, theindividual layer-like structures are separated from one another by theproduction of a mechanical strain within the respective porous layer orby an anodising process at a boundary surface of the respective porouslayer.
 13. Method in accordance with claim 12, characterised in that,after the separation of the individual layer-like structures andoptionally after the removal of residues of the porous layer, furtherstructures are produced on the one and/or other free surface of therespective layer-like structure.
 14. Method in accordance with claim 12,characterised in that, prior to the separation of the individuallayer-like structures from the multiple structure, these arerespectively provided with a carrier layer or secured to a carrier. 15.Method in accordance with claim 12, 13, or 14, characterised in thatrespective further structures are grown by epitaxial methods onto thesurfaces of the so formed layer-like structures originally facingtowards the substrate.
 16. Method of manufacturing a substrate forsemiconductor epitaxy, characterised in that one generates or applies alayer having hollow cavities or a porous material layer onto or out of afirst substrate, the layer optionally having a structured free surfacewhich, for example, has grooves arranged parallel to one another, inthat one applies a second substrate onto the free, optionally structuredsurface of the porous material layer and subsequently separates thesecond substrate from the first substrate using the porous layer as aposition of intended fracture by the production of a mechanical strainsuch that a layer or sections of the porous material layer remains orremain adhered to the second substrate, whereby the second substrate canbe used for epitaxial methods.
 17. Method in accordance with claim 16,characterised in that, following the separation of the second substratefrom the first substrate, the remainder of the porous layer is removedfrom the first substrate, a new porous layer is produced on the firstsubstrate and the process of claim 20 is repeated, with it beingpossible to repeat this method several times, in order to produce aplurality of second substrates starting from a first substrate. 18.Method in accordance with claim 16 or 17, characterised in that theapplication of the second substrate to the first substrate takes placeby means of an adhesive.
 19. Method, in particular in accordance withone of the preceding claims, characterised in that a substrate isproduced, in particular on an insulated carrier material having asurface which is covered with sections of porous semiconductor material,i.e. has regions which are free of porous material, for example inaccordance with the second substrate which arises in accordance withclaims 16 to 18 after the separation from the first substrate; in thatthe free surface of the (second) substrate covered with sections of theporous material is covered with a layer of amorphous silicon and theamorphous silicon is converted by a subsequent thermal treatment atpositions where it covers the sections into monocrystalline silicon, sothat a desired pattern of amorphous silicon and monocrystalline siliconis present on the (second) substrate, for example for the production ofa flat screen.
 20. Method for the manufacture of a substrate comprisinga carrier layer and a porous layer of silicon or of anothersemiconductor material provided thereon, characterised in that acylindrical bar consisting of a single crystal semiconductor material,for example p- or n-Si, is continuously treated at its surface toproduce a porous surface layer, for example in that the jacket surfaceof the bar is dipped during a rotation about the cylinder axis into anHF bath and an electrical potential drop is produced with correspondingflow of current from the bar to an electrode arranged in the HP bath,while the porous surface layer which is produced is continuously drawnoff from the bar, for example by a carrier layer which is continuouslyapplied to the surface, and in that the layer-like structure issubsequently grown onto the surface layer, in particular onto the freesurface of the drawn off surface layer opposite to the carrier layer.21. Method in accordance with claim 20, characterised in that the drawnoff layer is brought into a tube shape and subsequently converted by anepitaxial method into a monocrystalline tube.
 22. Method in accordancewith one of the preceding claims, characterised in that the generationof the mechanical strain acting within the porous layer which leads tothe separation of the layer-like structure, or of a part of it, from thesubstrate is produced by one of the following methods: a) by lifting thelayer-like structure from the substrate, b) by an ultrasonic treatment,c) by the generation of strong thermal gradients, for example by currentflow through the porous layer or by illumination from one side, or d) byexpansion or change of state (from the liquid phase to the vapour phase,from the liquid phase to the solid phase, for example by theintroduction of water) of a fluid (gas or liquid) or solvent filled intothe pores of the porous layer.
 23. Method of manufacturing layer-likestructures in which a material layer having hollow cavities, preferablya porous material layer, is produced on or from a substrate consisting,for example, of monocrystalline p-type or n-type Si, characterised inthat an uppermost layer of the cavity exhibiting or porous layer, ismelted, at least in places, for example by means of a laser beam, anelectron beam, or a focused light beam, and is subsequently caused tosolidify for the production of a single crystal non-porous layer and thesolidified layer is subsequently separated from the substrate,optionally after the growth of a layer-like structure thereon using thecavity exhibiting or porous layer, as a position of intended separation,for example by the production of a mechanical strain within the cavityexhibiting or porous layer, or at a boundary surface of the cavityexhibiting or porous layer.
 24. Method of manufacturing layer-likestructures, in particular m accordance with one of the preceding claims,in which a layer-like structure (of a semiconductor material) whichoptionally consists of only one layer, is formed on a porous substrateof p-type or n-type Si by an epitaxial method or by another depositionmethod, or by the fusing of the surface of the porous substrate, whereinthe porous substrate consists either fully of porous material or can bepresent in the form of a porous layer on a carrier of the same materialor of a different material and wherein the total structure is dippedinto an etching solution containing HF for the separation of thelayer-like structure from the porous layer or from the porous substrate,the etching solution penetrates the porous material and holes are madeavailable, for example by means of illumination or by the application ofa voltage, for example between the etching solution and the layer-likestructure, whereby the porosity is increased in the boundary surfaceregion between the layer-like structure and the porous layer for theseparation of the layer-like structure from the substrate.
 25. Method inaccordance with claim 24, characterised in that the initial porosityamounts to 5 to 50%, preferably to 10 to 20% and is preferably increasedto 100% for the carrying out of the separation.
 26. Substrate, inparticular of single crystal semiconductor material and having a porousmaterial layer on the surface of the substrate, characterised in thatthe free surface of the porous material layer has a structuring. 27.Substrate in accordance with claim 26 in combination with a layer-likestructure grown onto the surface of the porous layer by an epitaxialmethod (homo-epitaxial method or hetero-epitaxial method).
 28. Substratein accordance with claim 26 in combination with a second substrateadhered to the structured surface of the porous layer.
 29. Substrate inaccordance with claim 28, characterised in that the bond between thesecond substrate and the porous layer is realised by an adhesive, by abonding process, or by a diffusion brazing process, or by an epitaxialprocess.
 30. Substrate of any desired solid material having sections ofa porous single crystal semiconductor material bonded to at least theone surface of the substrate, with the crystal orientation in eachsection being at least substantially the same.
 31. Substrate inaccordance with claim 30, characterised in that a layer of amorphoussilicon is applied to it and is optionally converted at the positionswhere it covers the sections consisting of porous single crystalsemiconductor material into single crystal material, with the substratepreferably being used in a flat screen.
 32. Substrate in accordance withclaim 27, characterised in that the surface of the layer-like structureremote from the substrate consists of a single crystal semiconductormaterial with the same structuring as the previously free surface of theporous layer of the substrate, with it also being possible to drop thisstructuring, i.e. it can also be planar surface, and in that the surfaceis likewise realised as a porous layer with a further layer-likestructure arranged on this porous structured layer, with the furtherlayer-like structure preferably being the same as the first layer-likestructure and with this structure repeating as often as desired. 33.Substrate comprising a strip of flexible solid material with a strip ofporous single crystal semiconductor material on the surface of thestrip-like substrate.
 34. Substrate in accordance with claim 33,characterised in that the porous layer is strained.
 35. Substrate inaccordance with one of the preceding claims 26-34, characterised in thatthe porous layer or the porous layers and at least a part of thelayer-like structure consists of n-Si or p-Si or of any desiredsemiconductor material or of any desired compositional semiconductor,for example InP.
 36. Photocell comprising a transparent plate,preferably of glass, a layer-like structure beneath it, in particular ofSi with at least one structured surface having light traps, a p-njunction and also contacts to the p-type and n-type Si and a reflector,characterised in that the silicon is monocrystalline silicon, in that anelectrode is provided between the transparent plate and the silicon ofthe one conductive type (p-type or n-type), preferably a grid electrode,in particular a transparent electrode, and in that Si of the otherrespective conductive type is arranged on the side of the Si of thefirst named conductive type remote from the transparent plate and on thereflector.
 37. Photocell in accordance with claim 36, characterised inthat the layer-like structure comprises a layer of p-Si and a layer ofn-Si, with the n-Si layer being arranged beneath the transparent plateand above the p-Si layer.
 38. Photocell in accordance with claim 36,characterised in that the layer-like structure consists of an n-Silayer, in that the reflector consists of aluminum and through diffusioninto the n-Si layer converts this into p-Si.
 39. Radiation detectorcomprising a substrate with a plurality of recesses arranged in thissubstrate, a layer of a semiconductor material which is arranged abovethe substrate and lines and covers the recesses, a transparent platewhich covers over the recesses and also piezoelectric sensors whichdetect bending of the membrane formed by the layer of semiconductormaterial arising as a result of the incident light, with the individualrecesses being designable for respective radiation wavelengths, forexample through filters integrated into the transparent plate or mountedthereon.
 40. Method of producing a semiconductor circuit, for example aseries connection of a plurality of solar cells, in particular using oneof the substrates manufactured in accordance with the claims 1-35 and/orfor the realisation of a photocell in accordance with one of the claims36-38, characterised in that a shadow mask is arranged in front of thesubstrate during the layer manufacture and transverse to the transportdirection of the atoms to be deposited and is used to control layergrowth.
 41. Method in accordance with claim 40, characterised in thatduring the layer growth or between the growth of individual layers arelative displacement is effected between the shadow mask and thesubstrate, in particular parallel to the substrate, in order, forexample, in a first step, in a first position of the shadow mask, toproduce a trench between layer regions of the first conduction type and,in a subsequent step, in a further position of the shadow mask, toproduce an overlap between an edge region of a further layer of anotherconduction type and an exposed edge region of the layer of the firstconductive type present adjacent to the trench, whereby the seriesconnection is produced between two edge regions, i.e. between twosemiconductor elements formed by the layers on both sides of the trench.42. Method in accordance with one of the claims 40 to 41, characterisedin that a shadow mask formed from wires of any desired cross-sectionalshape is used, which is optionally moved, for example periodicallymoved, from side of the substrate holder to the other, in order toposition new wire lengths forming the shadow mask in front of thesubstrate in each case.
 43. Semiconductor structure, in particularproduced in accordance with one of the claims 40 to 42, characterised byfirst and second regions of a layer of a first conductivity type whichare separated from one another by means of a trench, by first and secondregions of a further layer, optionally of another conductivity type,which is either deposited directly above the first named layer orseparated from the latter by at least one further layer, and by an edgeregion of the first region of the further layer which crosses over thetrench and directly overlaps an edge region of the second region of thefirst named layer.